OS X Mavericks is available in the Mac App Store now, but it's only an upgrade—but if you want to do a clean install, you'll need to manually burn an installation flash drive. Here's how to do it.

macOS Monterey version 12 is a stable, lightweight upgrade that works great on older Macs, and you can use the direct DMG or ISO links to download, install, upgrade, or downgrade your system easily.